Integrated circuits are mass produced by fabricating hundreds of identical circuit patterns on a single semiconductor wafer. One of the many different processes repeated over and over in manufacturing these integrated circuits is that of using a mask and etchant for forming a particular feature. In such a mask and etching process, a photo mask containing the pattern of the structure to be fabricated is created, then, after formation of a material layer within which the feature is to be formed, the material layer is coated with a light-sensitive material called photoresist or resist. The resist-coated material layer is then exposed to ultraviolet light through the mask, thereby transferring the pattern from the mask to the resist. The wafer is then etched to remove the material layer unprotected by the resist, and then the remaining resist is stripped. This masking process permits specific areas of the material layer to be formed to meet the desired device design requirements.
In the etching process described above, it is important that the etching selectively remove the unwanted material and that the material underlying the material layer is not excessively damaged. A common way to accomplish this is to deposit or otherwise form an etch stop layer on the wafer prior to formation of the material layer. Such etch stop layers are commonly made of a material that is resistant to the particular etching process used.
In the integrated circuit fabrication art, the property of being resistant to an etching process is called the “selectivity” of a material. The selectivity of a particular material in a particular etching process is usually defined as the etching rate of the material to be removed divided by the etching rate of the particular material. Thus, a material that is highly resistant to an etch is said to have a high selectivity.
One of the more common etch stop layers currently used in the fabrication of integrated circuits is a single layer of silicon nitride (Si3N4). Unfortunately, Si3N4 does not provide the desired amount of selectivity required in certain of today's desired applications. The industry has attempted to use a single layer of silicon-rich nitride (SixNy, where the ratio of x:y is equal to or greater than 1.0) to increase the selectivity required for these applications, however, it has done so with limited success. Interestingly, silicon-rich nitride is somewhat conductive as compared to conventional silicon nitride, and thereby introduces certain undesirable electrical characteristics, such as source-to-drain and plug-to-plug leakage.
Accordingly, what is needed in the art is an etch stop that does not experience, or in another aspect introduce, the problems that arise with the use of the prior art etch stops.